INSECTS. 227 



its division of the body. The blood is for the most part 

 white. 



386. The respiration differs in the different classes. In 

 the Crustacea, the class to which the Lobsters and Crabs 

 belong, the respiratory organs are, like those of the Fish- 

 es, fitted for the action of air mingled with water. They 

 are really gills in various forms. But in the Insects and 

 Spiders the respiration is aerial ; that is, the respiratory 

 organs are fitted for the action of air alone by itself. 



387. The symmetry of the body is even more complete 

 in the Articulates than it is in the Vertebrates. In the 

 latter there are many of the internal organs in regard to 

 which the two halves of the body are not alike, as the 

 heart, stomach, lungs, etc. But in the Articulates thi^ 

 symmetry is extended even to some of these internal 

 organs. 



388. The Articulata are commonly divided into seven 

 classes. 1. Insects. These have the three divisions of 

 the body, the head, the thorax or chest, and the abdo- 

 men. They have antennae or feelers on the head, three 

 pairs of legs, and generally one or two pairs of wings. 

 2. Myriapoda, the Centipedes. They have no separation 

 of the body into thorax and abdomen. The head, how- 

 ever, is very distinct. There are seldom less than twen- 

 ty-four pairs of legs. The segments of the body are nu- 

 merous and equal. 3. Arachnida, including the Spiders, 

 Scorpions, and Mites, are characterized by the union of 

 the head and thorax, by the very distinct separation of 

 this cephalo-thorax from the abdomen, by the possession 

 of four pairs of legs, and by their want of aritennaB. In 

 all these three classes the respiration is aerial. 4. Crus- 

 tacea, or Crabs, Lobsters, etc. The respiration is aquatic, 

 like that of Fishes. They have from five to seven pairs 

 of legs. The body is variously divided. In some the 

 three parts are distinct, as in Insects ; in others the ar- 

 rangement is like that of the Arachnida ; and in others 

 still it is like that of the Myriapoda. The classes already 



